The invention relates to the field of vapor phase chemical reactions. More particularly, the invention relates to a particular reactor design and reaction process for such reactions.
Many commodity chemicals, such as ethylene oxide, acrylonitrile, hydrogen cyanide, and titanium dioxide are produced via vapor phase reactions. The economics of these processes are largely determined by the reactant selectivity (the moles of product formed divided by the moles of reactant which undergo reaction), reactant conversion (the moles of reactant that react divided by the mole of reactant entering the reactor), and the capital cost of the reactor.
Conventional vapor phase reactors are designed and operated in a manner to maximize selectivity and conversion within a narrow window of operational parameters. Chief among these operational parameters are reactant concentrations, pressure, temperature, and velocity, with limits on these parameters set primarily by reactant or product flammability and capital cost restrictions. In particular, reactant feed concentrations must often be kept below the lower flammability limit and the resulting oxygen to hydrocarbon ratio controls oxidation selectivity and conversion.
In cases where pure oxygen alone is used at the optimum oxygen to hydrocarbon ratio, the oxygen-feed mixture is near stoichiometric, and therefore quite flammable. Thus, an added ballast gas such as methane, CO2 or steam is necessary as a diluent. The addition of a separate diluent gas is not necessary in air-based processes, as the nitrogen in air acts as the diluent. Unfortunately however, the use of either air or diluted oxygen results in decreased reaction efficiencies and increased reactor size.
At present there are three broad categories of commercial reactor designs that are based on how the reactants are contacted. These categories are pre-mixed, in-situ mixed, and indirectly contacted.
Pre-mixed reactors include fixed catalytic bed, catalytic gauze bed, and catalytic monolith. In pre-mixed systems, the reactants are separately conveyed to a device which mixes the two streams. The residence time within this mixing zone is typically on the order of seconds. The reaction mixture can be preheated by providing thermal energy to the mixture or to either one or both of the reactants prior to interacting. The uniform mixture flows to a reaction zone where the reaction is initiated. In most cases, the reaction zone contains a heterogeneous catalyst. The residence time within the reaction zone is on the order of one second for fixed bed reactors and 0.01 seconds for gauze bed and monolith reactors. In a few cases, the reaction is performed homogeneously without the aid of a catalyst. In these cases, the reaction zone may contain an ignition source or some flame stabilization mechanism, and the residence time in the reaction zone is on the order of 0.1 seconds.
The most common catalytic pre-mixed reactor is a fixed bed reactor. In this type of reactor, a solid catalyst is packed within long tubes which are placed within a circulating heat transfer fluid which is used to remove the heat of reaction. Products such as phthalic anhydride, maleic anhydride, acrylic acid, ethylene oxide, and vinyl acetate are made using these reactors.
Another commercialized catalytic pre-mixed reactor is a gauze reactor. In these reactors, the catalyst is a very thin (on the order of a centimeter) bundle of wire mesh through which the reaction mixture flows. No attempt is made to remove the heat of reaction from the catalyst bed. Hydrogen cyanide and nitric oxide are made using this type of reactor.
A catalytic monolith reactor is a solid porous structure through which gas can flow. Catalytic monoliths have been used commercially to perform total oxidation reactions in catalytic combustion and in automotive catalytic converters. Typically with these reactors, no attempt is made to remove the heat of reaction within the reactor itself. Additionally, this type of reactor has been employed at bench scale to perform partial oxidation reactions.
In-situ mixed reactors include fluid catalytic bed and homogeneous burner reactors. In in-situ mixed systems, the reactants are brought together for the first time within the reactor. The major commercial in-situ mixed reactor is a fluid bed reactor. In fluid bed reactors, the reactants are injected separately into a vessel which contains a large mass of circulating solid particles. The reactant gases cause the motion of the solid particles. Typically, these particles act as catalysts. These reactors are typically equipped with internal steam coils to remove the heat of reaction within the reactor. The residence time in these reactors are typically on the order of 10 seconds. Examples of products produced in fluid beds include acrylonitrile and maleic anhydride.
Occasionally non-catalytic homogeneous reactions are also conducted in in-situ mixed systems, as disclosed in U.S. Pat. No. 2,559,638, 2,934,410, 3,172,729, 3,403,001. In these reactors, the reactants are separately conveyed to a single zone where they undergo simultaneous mixing and reaction.
The third class of reactors are indirectly contacted reactors, such as a catalytic transport bed reactor. With indirect contact reactors, the reactants are never actually brought together. Instead, the oxygen source and reactant are isolated from each other in the reactor either spatially or temporally. The only known commercial reactor of this type is the transport bed. In this type of reactor, the solid catalyst is conveyed between two different isolated sections of the reactor. One reactant is injected into one section while the second reactant is injected into the other. A version of such a reactor has recently been commercialized for the production of maleic anhydride.
While other vapor phase reactor designs have been proposed, there still remains a need in the art to develop a safe, low capital cost reactor with the potential of achieving uniquely high selectivities and high conversions for vapor phase reactions.
It is therefore an object of the invention to provide a vapor phase reactor that ensures reactions having high selectivity and conversion.
It is a further object of the invention to provide such a reactor that is safe and has low capital and operation costs.
It is a further object of the invention to provide a process for using such a reactor.
A preferred embodiment of the invention comprises a reactor including a source of a first reactant gas that is at a temperature greater than 500xc2x0 C., a mixing chamber or zone in which said first reactant gas mixes with a second reactant gas to form a substantially homogeneous reactant gas mixture, but wherein substantially no reaction takes place and a reaction chamber or zone wherein said mixture undergoes vapor phase reaction.
The invention also includes a chemical vapor reaction process. The steps of a preferred process include:
a) feeding a reactant gas or inert gas at a first pressure to a first chamber;
b) heating said reactant gas or inert gas;
c) passing said heated gas through one or more nozzles to a second chamber that is at lower pressure than said first chamber;
d) injecting a second reactant that is either in a gaseous or supercritical state into said second chamber;
e) maintaining said first reactant gas and said second reactant in said mixing chamber for a time sufficient to form a substantially uniform mixture of said first reactant gas and said second reactant but wherein substantially no reaction between said first reactant gas and said second reactant takes place;
f) injecting said substantially uniform mixture into a third chamber wherein reaction of said mixture occurs.